The Long Term Evolution (LTE) standard defines multiple channel types to organize transmissions between a base station and a mobile terminal. Logical channels are characterized by the type of information transmitted, and transport channels are characterized by how the information is transmitted.
The set of logical-channel types specified for LTE includes:                Broadcast Control Channel (BCCH): BCCH is used for transmission of system information from the network to all mobile terminals in a cell. This is information that is repeatedly broadcast by the network and which needs to be acquired by mobile terminals in order for the mobile terminals to be able to access and, in general, operate properly within the network and within a specific cell. The system information includes, among other things, information about downlink and uplink cell bandwidths, uplink/downlink configuration in case of Time Division Duplexing (TDD), detailed parameters related to random-access transmission and uplink power control, etc.        Paging Control Channel (PCCH): PCCH is used for paging of mobile terminals whose locations on a cell level are not known to the network.        Common Control Channel (CCCH): CCCH is used for transmission of control information in conjunction with random access.        Dedicated Control Channel (DCCH): DCCH is used for transmission of control information to/from a mobile terminal. This channel is used for individual configuration of mobile terminals such as different handover messages.        Multicast Control Channel (MCCH): MCCH is used for transmission of control information required for reception of the MTCH (for MTCH, see below).        Dedicated Traffic Channel (DTCH): DTCH is used for transmission of user data to/from a mobile terminal. This is the logical-channel type used for transmission of all uplink and non-Multimedia Broadcast over a Single Frequency Network (MBSFN) downlink user data.        Multicast Traffic Channel (MTCH): MTCH is used for downlink transmission of Multicast Broadcast Multimedia Services (MBMS) services.        
The following transport-channel types are defined for LTE:                Broadcast Channel (BCH): BCH has a fixed transport format, provided by the LTE specifications. It is used for transmission of parts of the BCCH system information.        Paging Channel (PCH): PCH is used for transmission of paging information from the PCCH logical channel.        Downlink Shared Channel (DL-SCH): DL-SCH is the main transport channel used for transmission of downlink data in LTE. It supports key LTE features such as dynamic rate adaptation and channel-dependent scheduling in the time and frequency domains, hybrid Automatic Repeat Request (ARQ) with soft combining, and spatial multiplexing. DL-SCH is also used for transmission of the parts of the BCCH system information not mapped to the BCH. There can be multiple DL-SCHs in a cell, one per user equipment device (UE) scheduled in this Transmission Time Interval (TTI), and, in some subframes, one DL-SCH carrying system information.        Multicast Channel (MCH): MCH is used to support MBMS.        Uplink Shared Channel (UL-SCH): UL-SCH is the uplink counterpart to the DL-SCH, that is, the uplink transport channel used for transmission of uplink data.        Random Access Channel (RACH): RACH is used for random access.        
Logical channels are multiplexed and mapped to transport channels as shown in FIG. 1 for the downlink and FIG. 2 for the uplink. The information on a transport channel is then further processed by the physical layer before transmission over the air interface to the receiver.
The physical layer is responsible for scrambling, coding, physical-layer hybrid-ARQ processing, modulation, multi-antenna processing, and mapping of the signal to the appropriate physical time-frequency resources. The physical layer also handles mapping of transport channels to physical channels.
A physical channel corresponds to the set of time-frequency resources used for transmission of a particular transport channel and each transport channel is mapped to a corresponding physical channel. In addition to the physical channels with a corresponding transport channel, there are also physical channels without a corresponding transport channel. These channels, known as L1/L2 control channels, are used for Downlink Control Information (DCI), providing the mobile terminal with the necessary information for proper reception and decoding of the downlink data transmission, and Uplink Control Information (UCI) used for providing the scheduler and the hybrid-ARQ protocol with information about the situation in the mobile terminal.
The physical-channel types defined in LTE include the following:                Physical Downlink Shared Channel (PDSCH): PDSCH is the main physical channel used for unicast transmission, but also for transmission of paging information.        Physical Broadcast Channel (PBCH): PBCH carries part of the system information required by the terminal in order to access the network.        Physical Multicast Channel (PMCH): PMCH is used for MBSFN operation.        Physical Downlink Control Channel (PDCCH): PDCCH is used for downlink control information, mainly scheduling decisions, required for reception of PDSCH and for scheduling grants enabling transmission on the PUSCH (for PUSCH, see below).        Physical Hybrid-ARQ Indicator Channel (PHICH): PHICH carries the hybrid-ARQ acknowledgement to indicate to the terminal whether a transport block should be retransmitted or not.        Physical Control Format Indicator Channel (PCFICH): PCFICH is a channel providing the terminals with information necessary to decode the set of PDCCHs. There is only one PCFICH per component carrier.        Physical Uplink Shared Channel (PUSCH): PUSCH is the uplink counterpart to the PDSCH. There is at most one PUSCH per uplink component carrier per terminal.        Physical Uplink Control Channel (PUCCH): PUCCH is used by the terminal to send hybrid-ARQ acknowledgements, indicating to the eNodeB whether the downlink transport block(s) was successfully received or not, to send channel-status reports aiding downlink channel-dependent scheduling, and for requesting resources to transmit uplink data upon. There is at most one PUCCH per terminal.        Physical Random Access Channel (PRACH): PRACH is used for random access.        
The mapping between transport channels and physical channels is illustrated in FIG. 1 for the downlink and FIG. 2 for the uplink. Note that some of the physical channels, more specifically the channels used for downlink control information (PCFICH, PDCCH, PHICH) and uplink control information (PUCCH), do not have a corresponding transport channel.
The different steps of the DL-SCH physical layer processing are outlined in FIG. 3. To randomize the interference between cells, LTE uses (cell-specific) scrambling of the coded transport channel data prior to mapping to the time-frequency resources. The purpose of scrambling (or, in general, randomization) is to make a signal to appear as random “noise” to a receiver not applying the correct descrambling sequence. Randomizing the transmitted data is beneficial as it allows spatial reuse of transmission resources. Although the resources are separated in the spatial domain, the isolation will often not be perfect (commonly referred to as the transmissions not being perfectly orthogonal). Thus, transmissions in one area may interfere with transmissions in another area. To avoid the receiver demodulating the wrong transmission, it is beneficial to ensure that any interference appears as random noise at the receiver. This is a well-known principle and has been used in several cellular systems supporting frequency reuse between cells, e.g., Wideband Code Division Multiple Access (WCDMA)/High Speed Packet Access (HSPA), LTE, and Code Division Multiple Access 2000 (CDMA2000). Sometimes the term quasi-orthogonal transmission is used to refer to the situation when multiple transmissions are not perfectly isolated (in time, frequency, code, or spatial domains) but randomization has been used to reduce the impact from one transmission to another.
The remaining downlink transport channels are based on the same general physical-layer processing as the DL-SCH, although with some restrictions in the set of features used. The UL-SCH in the uplink also follows similar physical-layer processing although there are some, for this disclosure irrelevant, differences such as the used of Discrete Fourier Transform (DFT) precoding for the UL-SCH.
Orthogonal Frequency Division Multiplexing (OFDM) is the basic transmission scheme for both the downlink and uplink transmission directions in LTE although, for the uplink, specific means are taken to ensure efficient power-amplifier operation. In the time domain, LTE transmission is organized into (radio) frames of length 10 milliseconds (ms), each of which is divided into ten equally sized subframes of length 1 ms as illustrated in FIG. 4. Each subframe consists of two equally sized slots of length Tslot=0.5 ms with each slot consisting of a number of OFDM symbols including cyclic prefix.
A resource element, consisting of one subcarrier during one OFDM symbol, is the smallest physical resource in LTE. Furthermore, as illustrated in FIG. 5, subcarriers are grouped into resource blocks, where each resource block consists of 12 consecutive subcarriers in the frequency domain and one 0.5 ms slot in the time domain. Each resource block thus consists of 7×12=84 resource elements in case of normal cyclic prefix and 6×12=72 resource elements in case of extended cyclic prefix. Although resource blocks are defined over one slot, the basic time domain unit for dynamic scheduling in LTE is one subframe, consisting of two consecutive slots. The minimum scheduling unit consisting of two time-consecutive resource blocks within one subframe (one resource block per slot) can be referred to as a resource block pair. The resource block definition above applies to both the downlink and uplink transmission directions.
Downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. The LTE specification includes several types of downlink reference signals which are transmitted in different ways and used for different purposes by the receiving terminal.                Cell-Specific Reference Signals (CRSs) are transmitted in every downlink subframe and in every resource block in the frequency domain, thus covering the entire cell bandwidth. The CRSs can be used by the terminal for channel estimation for coherent demodulation.        Demodulation Reference Signals (DM-RSs), also sometimes referred to as UE-specific reference signals, are specifically intended to be used by terminals for channel estimation for PDSCH when the CRSs cannot be used. The label “UE-specific” relates to the fact that each demodulation reference signal is intended for channel estimation by a single terminal. That specific reference signal is then only transmitted within the resource blocks assigned for PDSCH transmission to that terminal.        CSI Reference Signals (CSI-RSs) are specifically intended to be used by terminals to acquire Channel-State Information (CSI) in case when        
DM-RSs are used for channel estimation. CSI-RSs have a significantly lower time/frequency density, and thus implies less overhead, compared to the CRSs. A terminal can be provided with information about multiple CSI-RSs, one to measure upon and one or several that the terminal shall treat as “unused” resource elements (CSI-RS muting).                MBSFN reference signals are intended to be used for channel estimation for coherent demodulation in case of MCH transmission using MBSFN.        Positioning Reference Signals (PRSs) were introduced in LTE release 9 to enhance LTE positioning functionality, and more specifically to support the use of terminal measurements on multiple LTE cells to estimate the geographical position of the terminal. The positioning reference symbols of a certain cell can be configured to correspond to empty resource elements in neighboring cells, thus enabling high-Signal-to-Interference (SIR) conditions when receiving neighboring cell positioning reference signals.        
There are two types of reference signals defined for the LTE uplink:                Uplink DM-RSs are intended to be used by the base station for channel estimation for coherent demodulation of the uplink physical channels (PUSCH and PUCCH). DM-RSs are thus only transmitted together with PUSCH or PUCCH and are then transmitted with the same bandwidth as the corresponding physical channel.        Uplink Sounding Reference Signals (SRSs) are intended to be used by the base station for channel-state estimation to support uplink channel-dependent scheduling and link adaptation. The SRSs can also be used in cases when uplink transmission is needed although there is no data to transmit. Sounding reference signals can either be transmitted periodically as configured by higher layers or as “one shot” upon request from the network.        
Reference signals of different types, both in uplink and downlink, are typically separated in the time and/or frequency domain. For example, the downlink CRSs and DM-RSs from the same cell occupy different resource elements. These reference signals are therefore said to be orthogonal as no interference will occur between the two. However, between reference signals of the same type but belonging to different cells or different terminals, orthogonality can in general not be provided as this would result in excessive resource consumption. Therefore, the reference signal sequences and the processing in general is such that two reference signals use the same time-frequency resources but with different (pseudo-random) sequences to reduce impact from one reference signal to another. In essence, this idea of quasi-orthogonality is the same as scrambling for the data transmission.
Cell search is the process in LTE where the terminal acquires frequency and time synchronization to a cell and acquires the physical-layer cell identity (ID) of the cell (there are in total 3×168=504 possible identities). To assist the cell search, two special signals are transmitted on each downlink component carrier, the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). Although having the same detailed structure, the time-domain positions of the synchronization signals within the frame differ somewhat depending on if the cell is operating in Frequency Division Duplexing (FDD) or TDD mode. Time and frequency synchronization is required at the receiver (i.e., the UE) in order to properly receive and process any information transmitted by the transmitter (i.e., the base station).
The physical-layer cell identity of the cell in which the terminal has located is obtained by the terminal and is used for multiple purposes in LTE including:                for uplink transmissions:                    to determine the scrambling sequence used for the PUSCH uplink channel and the pseudo-random sequence used for the PUCCH uplink channel;            to determine the frequency hopping pattern for uplink transmission on PUSCH (if hopping is enabled);            to determine the sequence and, if enabled, sequence hopping pattern for uplink DM-RS; and            to determine the sequence and, if enabled, sequence hopping pattern for uplink SRS;                        for downlink transmissions:                    to determine the scrambling sequence for downlink unicast data transmission on PDSCH;            to determine the scrambling sequence for downlink broadcast of system information (PBCH and Broadcast Channel (BCH) mapped to PDSCH) and paging (PCH mapped to PDSCH); and            to determine the scrambling and time-frequency mapping of the PCFICH, PHICH, and PDCCH, used for transmission of downlink control information; and                        for downlink reference signals:                    to determine the sequence and the frequency location used for the CRSs;            to determine the sequence and, in some cases (antenna port 5) the frequency-domain location used for the UE-specific DM-RSs;            to determine the sequence and the frequency location used for the PRSs; and            to determine the sequence used for the CSI-RSs.Thus, as can be seen from the extensive list above, the physical-layer identity of the cell to which the terminal is connected influences many functions as seen in FIG. 6. In particular,                        uplink transmission,        downlink unicast reception, and        downlink broadcast receptionall use functions with parameters derived from the same physical-layer cell ID.        
The use of a so called heterogeneous deployment or heterogeneous cellular communication network is considered to be an interesting deployment strategy for cellular communication networks. As illustrated in FIG. 7, a heterogeneous deployment 10 includes a macro node 12 (i.e., a macro base station) and a pico node 14 (i.e., a pico base station) with different transmit powers and with overlapping coverage areas. Notably, a heterogeneous cellular communication network typically includes numerous macro nodes 12 and numerous pico nodes 14. In such a deployment, the pico nodes 14 are typically assumed to offer high data rates (megabits per second (Mbit/s)), as well as provide high capacity (users per square meters (users/m2) or Mbit/s/m2), in the local areas where this is needed/desired, while the macro nodes 12 are assumed to provide full-area coverage. In practice, the macro nodes 12 may correspond to currently deployed macro cells while the pico nodes 14 are later deployed nodes, extending the capacity and/or achievable data rates within a macro cell 16 served by the macro node 12 where needed. In a typical case, there may be multiple pico nodes 14 within the macro cell 16.
The pico node 14 of the heterogeneous deployment 10 typically corresponds to a cell of its own, i.e., a pico cell 18, as illustrated in FIG. 8 where the indices “p” and “m” indicate common signals/channels for the pico and macro cells 16 and 18, respectively. This means that, in addition to downlink and uplink data transmission/reception, the pico node 14 also transmits the full set of common signals/channels associated with a cell. In the LTE context this includes:                The PSSs and SSSs corresponding to the physical-layer cell ID of the pico cell 18,        The CRSs, also corresponding to the physical-layer cell ID of the pico cell 18. The CRS can, for example, be used for downlink channel estimation to enable coherent demodulation of downlink transmissions.        The BCH with corresponding pico cell system information for the pico cell 18.As the pico node 14 transmits the common signals/channels, the corresponding pico cell 18 can be detected and selected (connected to) by a UE.        
If the pico node 14 corresponds to a cell of its own, so-called L1/L2 control signaling on the PDCCH physical channel are also transmitted from the pico node 14 to connected UEs in addition to downlink data transmission on the PDSCH physical channel. For example, the L1/L2 control signaling provides downlink and uplink scheduling information and hybrid-ARQ related information to UEs within the cell.
In a heterogeneous deployment with a pico node corresponding to a cell of its own (FIG. 8), there is an inherent downlink/uplink imbalance due to the different transmit power of the macro and pico nodes/cells. This imbalance is illustrated in FIG. 9. The UE may connect to the cell (macro or pico) to which the path loss is the smallest. At least from an uplink data rate point-of-view, this is preferred as, for a given available UE transmit power, a smaller path loss leads to higher received power and thus to the possibility for higher data rates. However, due to the fact that common signals/channels as well as L1/L2 control channels are transmitted with higher power from the macro cell 16, compared to the pico cell 18, the UE connected to the pico cell 18 may experience very high interference from the transmission of these signals/channels in the macro cell 16. Although there are means to at least partly mitigate this interference, this requires special UE functionality not necessarily implemented in all UEs.
Alternatively, the UE may connect to the cell (macro or pico) from which the common channels (in practice the cell-specific reference signals) are received with the highest power. This is equivalent to say that the UE connects to the cell with the lowest path loss, weighted by the cell transmit power. However, due to the higher transmit power of the macro cell 16, a UE may then connect to the overlaid macro cell 16 even if the path loss to the pico cell 18 is smaller, leading to at least lower uplink data rates and potentially also a reduced downlink efficiency on a system level (although the downlink signals are received with stronger power from the macro cell 16, this is achieved at the expense of causing more downlink interference to other UEs).